A cloaking device was a form of stealthtechnology that used selective bending of light (and other forms of energy) to render a starship or other object completely invisible to the electromagnetic spectrum and most sensors. It was encountered in varying forms over the centuries.

During a battle near a Suliban helix in a class seven gas giant, Enterprise managed to capture a cell ship. Chief Engineer Charles Tucker III studied the device in his spare time, and in 2152 the ship was used to mount a rescue mission. Unfortunately, Tucker's experience with cloaking technology was somewhat limited and he accidentally doused his hand with the particles the ship used to generate the cloaking effect, temporarily rendering it invisible. (ENT: "Broken Bow", "The Communicator")

Enterprise learned how to penetrate a Suliban cloaking device in March 2152, when the temporal agentDaniels explained how to construct a quantum beacon so that Enterprise could reveal the location of the cloaked stealth-cruiser responsible for the destruction of Paraagan II. (ENT: "Shockwave")

Suliban cloaking devices seem to use a form of particle radiation to render objects invisible.

Romulan Bird-of-Prey cloaking

Enterprise was also responsible for an encounter with another organization that used stealth technology, the Romulan Star Empire. In April 2152, the ship accidentally entered a cloaked minefield protecting a Romulan-claimed planet. The quantum beacons proved effective against the minefield, but when two Birds-of-Prey suddenly arrived, Enterprise was not able to penetrate their more advanced cloaks. (ENT: "Minefield")

By the 23rd century, the very idea of practical invisibility was considered only theoretically possible, due to the enormous amount of power required. This was proven untrue in 2266, when the USS Enterprise encountered a Romulan Bird-of-Prey that used the device to cross the Romulan Neutral Zone and wipe out several Earth Outpost Stations. The Enterprise was able to defeat the Bird-of-Prey in battle, partially because the cloaked ship was still visible to tracking sensors, but there was no doubt the device itself had been a success. (TOS: "Balance of Terror")

In 2268, the Federation learned that the Romulans had developed a new and improved cloaking device; one that even tracking sensors could not pick up and that posed a threat to the security of the Federation. On stardate 5027.3, Starfleet Intelligence sent the Enterprise on a covert mission across the Neutral Zone to acquire the new cloak. The mission was a success, and Starfleet was able to procure an intact, modern Romulan cloaking device for study. The Federation also acquired a cloaking device from a captured Klingon Bird-of-Prey, dubbed the "HMS Bounty", in 2286. (TOS: "The Enterprise Incident"; Star Trek IV: The Voyage Home)

Gene Roddenberry indicated in various interviews that "our heroes don't sneak around", indicating that the Federation made a conscious decision to not develop cloaking technology.

Quark's cloaking device

In 2370, Quark illegally acquired a small cloaking device that was, as he put it, "not in the best of condition", but it "will work for about fifteen minutes." Initially intending to sell it, Quark instead installed it in the central engine core of a Cardassian shuttle, piloted by Natima Lang, to help the ship escape Deep Space 9 undetected. (DS9: "Profit and Loss")

Following the outbreak of hostilities with the Dominion in late 2370, the Romulan Empire allowed the use of one of their own cloaks aboard the USS Defiant. Although this was originally limited to usage within the Gamma Quadrant, CaptainBenjamin Sisko chose to ignore this limitation on occasion. In order to ensure that the cloak was used correctly, Subcommander T'Rul was assigned to the Defiant initially, during the ship's first mission with the cloak. In exchange, Starfleet had to provide all the intelligence it received on the Dominion. (DS9: "The Search, Part I", "The Search, Part II", "Visionary")

Ronald D. Moore commented: "We wanted to throw in a line somewhere explaining that there was a follow-on agreement with the Roms that allowed us greater freedom with the cloak, but never found a comfortable place to put it." (AOL chat,1997)

The use of a cloaking device on the holoship would seem to be a violation of the Treaty of Algeron.

A Klingon cloaking device

Although quite heavy, according to Rom, Klingon cloaking devices weigh less than Romulan cloaking devices. (DS9: "The Emperor's New Cloak")

It was even possible for an entire planetary body to be rendered invisible. The planetAldea was equipped with a powerful cloaking device that allowed itself to fully cloak. (TNG: "When The Bough Breaks")

Most vessels could not use their deflector shields when cloaked. Ships of the Klingon D12-class, for example, were vulnerable to attack for two seconds when their shields automatically dropped as the cloaking field formed. (TNG: "Face of the Enemy"; Star Trek Generations) Ships like the Scimitar with more advanced cloaking technology however did not have this problem, as it had primary and secondary shields that could remain online while the ship was cloaked. (Star Trek Nemesis)

Ever since the initial introduction of the cloaking device, there was an ongoing race between the development of new cloaking technologies and the development of new detection systems that could defeat them.

Quantum beacon deployed

Starfleet's earliest method of penetrating cloaks was developed in 2152, when Daniels provided Enterprise with 31st centuryquantum beacons so they could locate a Suliban stealth-cruiser. These devices also proved effective against the Romulan cloaking devices used on mines but were ineffective against the Bird-of-Prey cloaking devices. (ENT: "Shockwave", "Minefield")

The cloak used by the Romulan Bird-of-Prey that crossed the Neutral Zone in 2266 was less than perfect, allowing a starship to pick up a blip on its motion tracking sensors. This blip was not accurate enough for targeting of weapons and only appeared when the vessel moved, but it was enough to help the Enterprise locate the enemy vessel. By 2268, advances in Romulan technology had removed the problem, forcing Starfleet to steal a model of the new cloak. (TOS: "Balance of Terror", "The Enterprise Incident")

In 2371 it was not commonly understood by Starfleet personnel that cloaked ships radiate a slight subspace variance at warp speeds. These variances typically vanished once the vessel drops out of warp. (DS9: "The Search, Part I") If a cloaked ship exceeded its maximum propulsion capacity, it would not be able to fully cloak, and would appear on navigational sensors as a sensor echo. (TNG: "Tin Man")

Using modified cloaks, a combined fleet of Romulan and Cardassian warships attempted to conduct a sneak attack on Founders' homeworld in 2371. They determined that as long as their fleet traveled under the speed of warp 6 their warp signatures would remain undetected, even while under cloak. This modification, however, did not prevent the fleet from being detected by the sensors aboard Deep Space 9. In this case, the cloaked fleet appeared as high concentrations of tetryon particles. (DS9: "The Die is Cast")

Sensors aboard Deep Space 9 were also able to detect an entire nearby fleet of cloaked Klingon vessels jumping to warp as "a huge distortion wave in subspace", and Miles O'Brien was able to deduce their heading from the vector of the subspace disturbance. (DS9: "The Way of the Warrior")

Due to the enormous amount of power required to generate a cloaking field, there was, by and large, not enough power available to also power the weapons and shields. (TOS: "Balance of Terror") When the Romulan cloaking device was first installed on the USS Defiant, the ship even had to decloak to use the transporter. (DS9: "The Search, Part I") However, there were several times when advances in cloaking technology rendered these tactical inefficiencies untrue.

Prototype Bird-of-Prey, firing while cloaked

For example, in 2293, the Klingons developed a prototype Bird-of-Prey capable of firing photon torpedoes when cloaked. This ship, commanded by General Chang, was used to secretly attack Kronos One in such a way that it appeared the USS Enterprise-A was responsible, implicating Enterprise commanding officer James T. Kirk in the assassination of ChancellorGorkon. Fortunately for galactic peace, the Enterprise-A was able to deduce the existence of Chang's ship and devise a way to penetrate its cloak by tracking its plasma exhaust with new sensors for analyzing gaseous anomalies. The prototype ship was destroyed by the Enterprise-A and the USS Excelsior over Khitomer. (Star Trek VI: The Undiscovered Country)

In the script for Star Trek VI, the development of this cloaking device is said to have cost "millions".

Another advanced cloak was encountered in 2379. The Reman warbird Scimitar employed a new type of "perfect" cloaking device that did not give off any tachyon emissions or residual antiprotons, making the Scimitar completely undetectable while cloaked. It allowed the ship to fire weapons and use shields while cloaked, as well as allow the ship to drop the cloak protecting certain quadrants of the ship without de-cloaking the entire ship at once. The effectiveness of this technology was demonstrated when the Scimitar engaged the USS Enterprise-E, the IRW Valdore, and another warbird in the Bassen Rift, and was able to destroy one of the warbirds and cripple the other without suffering any significant damage. Only through repeated blind targeting, barrages of phasers and torpedoes, and the use of telepathic triangulation by Commander Deanna Troi, was the Enterprise-E able to overcome the Scimitar's cloak. (Star Trek Nemesis)

In addition, the mirror universeISS Enterprise was able to fire weapons while cloaked using a Suliban cloaking device. However, the cloak did appear to momentarily fail a few seconds after Enterprise launched torpedoes, perhaps indicating that the Imperial ship had insufficient power for simultaneous use of weapons and cloak. (ENT: "In a Mirror, Darkly")

The Treaty of Algeron did not stop Starfleet Intelligence from secretly and illegally developing the phasing cloak, a device which not only rendered a starship invisible, but also allowed it to pass through solid matter unimpeded. The device was lost in 2358 when the testbed vessel, the USS Pegasus, was presumed destroyed by a warp core breach in a mutiny by members of the crew concerned about the legality of the test. In 2370, the Pegasus was found not to have been destroyed and the existence of the illegal device was made public to the Romulans. (TNG: "The Pegasus")

In the early 2360s, the Klingon Empire experimented with interphase cloaking technology but the research was abandoned due to several accidents.

In 2368 the Romulans also unsuccessfully experimented with interphase cloaking technology, developing a prototype. The USS Enterprise-D had received a distress signal from the testbed vessel and CaptainPicard ordered an away team sent over to assist. During the mission, due to a malfunction in the device, EnsignRo Laren and Lieutenant CommanderGeordi La Forge were phase-cloaked during beam-out and rematerialized on different parts of the Enterprise. After attempting to bring them back, the crew gave up, believing them dead. Eventually, Ro and La Forge succeeded in revealing what had happened to them to the crew and Lieutenant Commander Data managed to reverse the effects of the cloak, bringing them back into phase. (TNG: "The Next Phase")

In 2372, a computer character manifesting itself as fear was offered a cloaking device as a means of keeping its program hidden from future intrusion. To the request, it replied "I already have a cloaking device, thank you anyway." The character's "cloaking device" immediately wrapped the character in a wool cloak. (VOY: "The Thaw")

There has been some controversy over the appearance of the cloaking device in Star Trek: Enterprise. TOS: "Balance of Terror" makes it reasonably clear that this is the first time such a device has been encountered, and early Enterprise episodes skirted very close to violating this by referring to the invisibility technology seen in ENT: "Broken Bow" and "Unexpected" as stealth technology; presumably this was intended to be a primitive precursor to cloaking, utilizing some method other than the selective bending of light.

Unfortunately, the episode "Shockwave" featured several explicit references to the Suliban stealth technology as cloaking. "Minefield" further complicated the problem by showing that the Romulans had cloaking technology in the 22nd century, making some possible justifications no longer workable. Unfortunately, there is no obvious way to reconcile the discrepancies, except to force an interpretation of Spock's words ("Invisibility is theoretically possible, captain – selectively bending light. But the power cost is enormous. They may have solved that.") to mean that the Federation thought invisibility was impossible because all previous forms of cloaking had been penetrated. Then again, Spock's notions regarding this particular matter may not represent the totality of Starfleet's expertise, as even he admits that he is not as qualified as Mr. Scott is when it comes to engineering. (TOS: "The Devil in the Dark")

Several staff writers on Star Trek: Enterprise – among them Producer Mike Sussman – believed it was a mistake to give cloaking technology to 22nd century Romulans. Season Four showrunner and Co-Executive Producer Manny Coto decided that Romulan ships would not have cloaks in any subsequent encounters. (Information provided by Mike Sussman)

In apocrypha the discrepancy is explained in the novel The Good That Men Do. The 22nd century Bird-of-Prey class ships were only used as testbeds for early prototype cloaking devices. After an antimatter-containment failure in one test systems, their use was abandoned and it took decades for the Romulans to develop a successful cloaking system.

There is also a running discrepancy as to whether or not the mirror universe has cloaking devices. While they have been seen in early mirror episodes of DS9, the last "mirror" episode featured a plot revolving around the "fact" that the technology doesn't exist in the mirror universe.

Several apocryphal sources, including Diane Duane's novel The Romulan Way, also credit Romulans with the development of the first cloaking technology, and state that the Klingons acquired it in trade with the Romulans, during their tenuous alliance.